On the Universality of the Relation Between Magnetic Fields and Star Formation in Galaxies

TL;DR

The paper tests whether a universal relation links magnetic field strength to star formation across galaxy mass, ISM phases, and energy budgets by analyzing 19 Azahar spiral disks with line-of-sight integrated maps of $B$, energy components, and star-formation metrics. Employing a full-physics MHD framework with SN-driven magnetic seeding, radiative transfer, and cosmic rays, the study shows a sub-linear $B$–SFR scaling with $\alpha \approx 0.2$–$0.3$ and a $B$–$\Sigma_{\rm SFR}$ slope near $1/3$, consistent with a SN-driven, turbulence-regulated small-scale dynamo and near-equipartition in active disks. Phase-resolved results reveal both CNM and WNM follow similar $B$–SFR trends, while magnetic energy fractions rise steeply with SFR, indicating magnetic fields approach dynamical importance in high-SFR environments. Model variations demonstrate that the dynamo saturation state, not merely initial seed strength, sets the observed B–SFR coupling, with SN-injection reproducing canonical slopes while strong primordial seeds saturate early and flatten the relation. Overall, the findings support SN-driven turbulence as the primary amplification mechanism, yielding a nearly universal B–SFR relation with meaningful implications for magnetic support and feedback in star-forming galaxies.

Abstract

The interstellar medium (ISM) is permeated by magnetic fields that affect gas dynamics and star formation. These fields correlate with supernova (SN)-driven turbulence, but whether the scaling is universal across galaxy properties, ISM phases, and energy budgets remains unclear. We quantify the dependence of magnetic fields on star formation activity including both regular and starburst galaxies. We analyse 19 spiral disks from the cosmological RTnsCRiMHD Azahar suite, deriving line-of-sight integrated maps to measure median magnetic-field strength ($B$), specific energies (thermal, turbulent, magnetic, and cosmic-ray), and star formation rate (SFR), star formation surface density ($Σ_{\mathrm{SFR}}$) and specific SFR (sSFR). We find an almost universal magnetic-field-SFR scaling with slope $α\approx 0.2$-$0.3$ across galaxy mass and ISM phases. The $B$-$Σ_{\mathrm{SFR}}$ slope ($α\approx 1/3$) supports an SN-driven, turbulence-regulated origin. Neutral gas is generally turbulence-dominated and in near equipartition with magnetic energy for systems with sSFR $\gtrsim 0.1$ Gyr$^{-1}$ and SFR $\gtrsim 1$ $M_\odot$ yr$^{-1}$. The simulated trends match observations with similar slopes ($α\approx 0.25$-$0.35$), indicating that SN-driven turbulence is the main amplification mechanism behind the near-universal $B$-SFR relation.

Figures (9)

• Figure 1: Multi-panel view of the simulated galaxy Azahar-a. The large left panel shows the rest-frame mock optical (SDSS $ugr$) image of the system. The two upper-right panels display the gas surface density and the star formation rate surface density, averaged over the past $100$ Myr. The three bottom panels show the total magnetic field strength for the full gas, CNM, and WNM components. White contours in all panels mark the disk region adopted for the analysis.
• Figure 2: Magnetic field strength as a function of global star formation and mass indicators. Each panel shows the median magnetic field strength $B$ within the disk mask of the simulated sample (green solid circles) measured as described in Section \ref{['The simulated galaxy sample']}. Error bars show IQR range. The solid green line shows the best fit, and the shaded region indicates the $95\%$ prediction interval, with their fitted slopes and the standard error reported in the legends. Samples of observed galaxies taken from the literature and their best-fit relations (dashed lines), are overlaid with different color crosses: red Lacki2013, black Lopez-Rodriguez2023, purple Chyzy2011, navy blue Beck2019, and goldenrod (Heald2022Stein2020Stein2019Mora-Partiarroyo2019). Their respective best-fit relations are shown as dashed lines. Upper limits from Chyzy2011 are shown as downward arrows, and are excluded from the fits. Top:$B$ versus integrated SFR over the last 100 Myr. Central:$B$ versus the SFR surface density. Bottom:$B$ versus sSFR.
• Figure 3: Magnetic field strength as a function of stellar mass $M_{\star}$ (solid green circles). The solid green line shows the best fit, and the shaded region indicates the $95\%$ prediction interval, with the fitted slopes and the standard error reported in the legend. Samples of observed galaxies taken from the literature and their best-fit relations (dashed lines), are overlaid with different colour crosses: red Lacki2013, black Lopez-Rodriguez2023, purple Chyzy2011, navy blue Beck2019, and goldenrod (Stein2019Mora-Partiarroyo2019Stein2020Heald2022). Their respective best-fit relations are shown as dashed lines.
• Figure 4: Specific energies as a function of the SFR (top panel) and sSFR (bottom panel). Each point represents the specific energy associated to the magnetic (blue), thermal (grey), turbulent (red) and cosmic ray (yellow) component within the disk of each galaxy. The best fits for each energy component are shown as coloured lines, with their corresponding slopes and standard errors provided in the legends.
• Figure 5: Magnetic field strength versus global star formation for neutral ISM phases. Panels show the median magnetic field strength $B$ within the disk mask of the simulated sample (solid circles) in the CNM (blue) and WNM (orange) phases. Error bars for each data point show the IQR for each galaxy. Solid lines indicate best-fit power laws, and the shaded region indicates the 95% prediction interval; fitted slopes are reported in the legends. Left:$B$ versus integrated SFR. Right:$B$ versus specific SFR (sSFR).